Hello, Canker sores are usually caused by viruses. Less commonly they can be part of other disease processed such as autoimmune disease (unlikely in a small child). I m not aware of canker sore being caused by food intolerance/sensitivity/allergy such as tomato. Canker sore can also be a sign of some vitamin or mineral deficiencies. Given that a very common cause of canker sore is a virus, canker sore are often associated with other symptoms such as sore throat, ear ache, mildly swollen lymph nodes, headache or low grade fever; such as in the case of your daughter. Although they can be very painful, canker sore usually resolve on their own. Hydrating well with water helps a lot and avoiding eating acidic or spicy foods (such as citrus, tomato sauce, garlic etc) is recommended. Hope I have answered your question. Let me know if I can assist you further. Regards, Dr. Kamila Seilhan, Internal Medicine Specialist